In a motor grader 10 that is a working machine, a drawbar 16A is swingably mounted to a front end portion of the motor grader 10, and a blade 12A is supported slidably in a lateral direction by a circle 14A rotatably mounted to the drawbar 16A. The circle 14A has a blade support 14B to support the blade 12A. The circle 14A has an integrally formed circle gear (not shown). To rotate the blade 12A, the motor grader 10 moves the circle gear by a pinion gear (not shown) to rotate the blade 12A in a left and right direction with respect to a forward traveling direction of the motor grader 10, thereby performing a desired operation. However, a rotational angle of the blade 12A is an angle in a predetermined range, for example, in a range of 90 degrees in most cases, and therefore the circle gear is worn and broken at a specific spot. When the circle gear is worn and broken, the entire circle 14A has to be replaced and tremendous replacement cost and replacing time are required.
In order to solve the problem, for example, patent document 1 (the U.S. Pat. No. 5,667,020) discloses a circle structure of a motor grader. Namely, as shown in FIG. 9 to FIG. 11, the drawbar 16 has a tubular frame 20 and a circular frame 18. The circle structure of the motor grader, in which the circle 14 is rotatably mounted to the circular frame 18, and a plurality of ring gear segments 30 are included in the circle 14 is disclosed.
In the circle structure of patent document 1, six of the same ring gear segments 30 are attached to a circular frame portion 26 to define a ring gear at 360 degrees. The ring gear segment 30 is constituted of 60 degrees of a gear span at 360 degrees. Each of the ring gear segments 30 is fixed to the circular frame portion 26 with one positioning bolt 32 and six fixtures 34. The bolt 32 positions the ring gear segment, and the entire area of the ring gear segment 30 responds to a shearing load. The fixtures 34 give fastening force to these ring gear segments 30 to fix them to the frame portion 26. Eight shoe assemblies 36 are mounted to the drawbar 16 in a circular array and support the circular frame portion 26 against the circular frame 18 of the drawbar 16. Such a circle structure can be repaired by removing the adversely affected ring gear segment 30 and inserting another ring gear segment in a predetermined spot without replacing the entire circle.
However, in the example of patent document 1, the circle gear is divided into the same six ring gear segments 30 and mounted to the circular frame portion 26, and therefore the worn or broken ring gear segment 30 has to be replaced. Accordingly, the ring gear 30 for replacement has to be prepared, which causes the cost, and raises the maintenance cost. If the worn or broken portion occurs across the two ring gear segments 30 at a border portion of two of the ring gear segments 30, the two ring gear segments 30 have to be replaced and the replacement cost is further added.
Since the circle gear is divided into the same six ring gear segments 30, the load applied to teeth of the ring gear segment 30 is received by the one ring gear segment 30. Therefore, this circle gear is insufficient in rigidity as compared with the circle gear which is not divided. Accordingly, deformation of the ring gear segment 30 becomes large, which reduces durability. Since the ring gear segment 30 is fixed to the frame portion 26 only by the one positioning bolt 32 and the six fixtures 34, it can respond to a shearing load with only the one ring gear segment 30. Therefore, as compared with the case in which the entire circle gear, which is not divided, responds to the shearing load, the fixing force with respect to the shearing load is insufficient, the ring gear segment 30 easily moves, and as a result, durability is reduced.
Since the ring gear segment 30 is only placed on a flat top surface 26U of the frame portion 26, a positioning bolt for the ring gear segment 30 and the frame portion 26 is needed. Since the circle gear is divided into the same six ring gear segments 30, even if a worn or broken ring gear segment 30 is replaced and reassembled for repair to make the ring gear, circularity is difficult to obtain and the entire ring gear is inferior in precision to make tooth contact unfavorable, which results in reduced durability.